Ferromagnetic material



United States Patent 3,117,935 FERRGMAGNETIC MATEREAL Poul Bernard Brann and Wim Kwestroo, Eindhoven,

Netherlands, assignors to North American Philips Company Inca, New York, N.Y., a corporation of Delaware No Drawing. Fiied .luue 6, 1961, Ser. No. 115,084

Claims priority, application Netherlmds June 23, 1950 Claims. (Cl. 252-625) The invention relates to a ferromagnetic material, in particular for use in magnet cores for high-frequency systerns.

In the system CaO-Fe G several compounds are known, for example, the compounds CaFe O Ca l-"e O' and CaFe O which are not ferromagnetic. From Journal of the American Chemical Society, vol. 81, No. 15,

page 3842 (1959), it is known that by the reaction of Fe O in a melt ar" CaCl two phases were obtained, i.e., weak-1y ferromagnetic needles and prisms, which proved to have the crystal structure and the composition of CaF-e O and hexagonal transparent plates having a composition Ca Fe O These plates were sufiiciently ferromagnetic for them to be separated from the first fraction. The compound was hexagonal and looked very similar to magneto-plumbite. In Nature, vol. 185, No. 4713, page 604 (1960), the compound 3CaO.FeO.7Fe O or Ca Fe +Fe +O is referred to, which is non-magnetic.

The material according to the invention consists of mixed crystals of compounds having a chemical composition according to the formula where D is at least one of the metals Y, Nb, Sm, Eu, Gd, Tb, Dy, Ho, Tm, Er, Yb, Lu, Bi and In, Z is at least one of the metals Th and Ce and in which a+b+c+d 0 which have a rhornbohedral crystal structure, the unit cell of which can be described in the hexagonal crystal system by a c-axis of about 95.0 A. and an a-axis of about 6.0 A.

In the crystals of the materials according to the invention, the spontaneous magnetization extends at right angles to the hexagonal axis and hence parallel to basal plme of the crystal. Thus, the crystal has a so-called preferred plane of magnetization. In this plane, the direction of magnetization is far more readily rotatable than in a direction not situated in this plane. Consequently, the materials have soft-magnetic properties. The initial permeability has values which are sufiiciently high for them to be of interest for use in electrical technology.

The metals represented by D have the common property that the radius of their trivalent ions lies between 0.92 A. and 1.15 A. Similarly, Th and Ce have the property that the radius of their tetravalent ions lies between 0.92 A. and 1.15 A.

The materials according to the invention may be produced by heating a correctly proportioned, finely powdered mixture or" the constituent metal oxides to a temperature between about 1100 C. and 1240" C., preferably between 1170 C. and 1210 C. Obviously, one or more of the constituent metal oxides may be replaced entirely or in part by compounds which, when heated, are converted to metal oxides, for example carbonates, oxalates and acetates. Furthermore, the constituent metal oxides may be replaced entirely or in part by at least one reaction product of at least two or the constituent metal oxides, for example CaFe O The term correctly proportioned is to be understood to signify that the proportions of the amounts of the metals in the initial mixture are substantially equal to those in the compounds to be produced.

If desired, the finely powdered starting mixture may first be presintered at a comparatively low temperature (800 C.-1100 C.), the reaction product being ground and the resulting powder being sintered again, and this sequence of operations may be repeated once or several times. In order to promote sintering, sintering agents, such as silicates and fluorides, may be added.

Bodies consisting of the ferromagnetic materials described may be obtained by sintering the starting mixture of the metal oxides or the like immediately in the desired shape, and also by pulverizing the reaction product of the presintering process, the resulting powder, if required after the addition of the binder, being shaped in the required form and sintered.

It will be clear that in the method described small amounts of impurities may readily be incorporated in the reaction product. Examples of these impurities are Fe G CaFe Or, and compounds having garnet structure.

Obviously, single crystals of the materials according to the invention may also be produced. In view of the incongruous melting points of the materials, one starts from a mixture having a composition different from that of the desired compound, that is to say, from a composition which is richer in CaO by 15 to 20 mol percent. The mixture is heated to a temperature such that it is entirely melted (1240 C.1280 C.). During subsequent slow cooling, single crystals of the relevant compound are formed. Preferably, cooling through the first 50 takes at least 2 hours.

It should be noted that, apart from the difference in composition, there are two more differences between the materials according to the invention and the compounds Ca Fe +Fe +O known from Nature, vol. 185, No. 4713, page 604 (1960). Firstly, the new materials are ferromagnetic, whereas the known material is stated to be non-magnetic, and secondly the new materials have a rhombohedral crystal structure the unit cell of which can be described in the hexagonal system by a c-axis of about 95.0 A. and an a-axis of about 6.0 A., whereas experiments have shown that the known material has a hexagonal crystal structure the unit cell of which can be described by a c-axis of about 31.5 A. and an a-axis of about 6.0 A.

EXAMPLE I A mixture of CaCO Fe O and Y O having a composition according to the formula Ca -Fe Y O was mixed with ethyl alcohol in a ball mill for 3 hours. After drying, the mixture was pre-fired in air at 800 C. for 16 hours, the reaction product then being ground with ethyl alcohol in a ball mill for 1 /2 hours. After drying, a small amount of a solution of an organic binder, nitrocellulose, was added to the product, which was then compressed to form rings, which were fired in oxygen at 1190 C. for 4 hours. X-ray examination showed that the rings consisted substantially entirely of crystals having the desired structure. For this compound a:0; b:0; c:O.6; (1:0; while D represents the metal Y.

Rings having different compositions were made in the same manner, and X-ray examination showed that they all consisted substantially entirely of crystals having the desired structure. The starting mixtures consisted of CaCO Fe O and In O having a composition according to the formula CaCG Fe O and Gd O having a composition according to the formula Ca4F Gd O25 CaCO Fe O and Nd O having a composition according were made in the same manner.

Starting from a mixture of 25 g. of CaCO 58.4 g. of

Fe O and 5 28 of Th0 (co r ondin o 39 l e to the formula 2 3 e- 2 1 m g mo per cent of Cal), 58 mol percent or Fe O and 3 mol percent a4 1a9 05 25 6:01; (1:0; DINd) of ThO crystals having a composition CaCO Fe O and T110 having a composition according c i 'fh o 0; 5 9; 9; 5 02; 31

to the formula 10 Were made 1n the same manner.

3.6 13.6 0.5 25 Z7111) The saturation magnetization of all these crystals was c cg F6203 and C802 having a Composition according 22 gauss. cc./g. at room temperature, While the Curie temto the formula perature was 150 C.

D What is claimed is:

35 13.6 0.5 25 1 6:0, zzce) 1O 1. A ferromagnetic material consisting essentially of C80, F6 0 and Bi O having a composition according mixed crystals of compounds having a chemical compoto the fo l sition according to the formula Ca Fe Bi O (a=0.4; 17:0; 0:0.3; d=0; D=Bi) 4-a-b 1-icd 2/3a+c 1/2b+3/4d 25 In the last case, the rings were not fired at 1190 C, 20 Where D is at least one metal selected from the group conbut at1140 C, sisting of Y, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Tm, Er,

A number of properties of these rings were measured. L i and Z is at 1638i 0116 m l t d fr m The following table shows the resistivity expressed in $2 the group wfisisfing 0f Th and Ce and Where cm. and the initial permeability and the loss factor tan 6 F 056130] at various frequencies. Owing to the electro-magnetic 5 2 losses, there is a phase difference between the field strength 5 5 1d the induction. Therefore, the initial permeability is g g usually represented as a complex quantity. The table F f is at least {16 gives the value of the real part t of the initial permen ability. The loss factor tan 6 is equal to an ,u"/;t' where said mixed crystals having a rhombohedral crystal strucis the imaginary part of the initial permeability. ture the unit cell of which can be described in the hexa- Table 1 mc./s 5 mc./s 70 mcJs. 100 rue/s. n in.

p tan 5 [LI tan 5 p tan 6 n tau 6 Game/13.41 030 15x10 1 10.5 0.03 10.0 0. 3.9 0.27 8.3 0.35 21.3 0.09 21.4 0.31 9.5 0.43 8.6 0.51

EXAMPLE II gonal crystal system by a c-axis of about 95.0 A. and an r zz-axis of about 6.0 A.

A mlxture of cage, YZOS and 2. A method of producing a ferromagnetic material as of F6203 (Correspcndmg to 40 percent of defined in claim 1 in which a mixture of oxides of said 2 mol Percent [203 and,58 m Parcel of F6203) metals in proportions corresponding approximately to was heated m Platmum crucible an Oxygen i those in said formula and in finely-divided form are Phere- Th6 fixture began to melt at about 1220 heated to a temperature of about 1100 C. to 1240 C. to It was heated to 1280 C. and subsequently cooled slowly. form Said composition Coohng Was performed 3 hours and Subse' 6O 3. A method of producing a single crystal consisting esqunt Coohng to room tempera? 1n 6 h A few sentially of a ferromagnetic material as defined in claim 1 crystals had fonPedi the cqmpgsltlon of Whlch was in which a mixture having a composition corresponding to chemical analysls and the Structure by that of said material and an excess of about 15 to 20 mol examinanon. The crystal structure was rhombohedral percent of cao is heated to a temperature of about 12400 and the unit cell could be described in the h xagonal (31012300 and Subsequently slowly Cooled crysial system by of about an 4. Amethod of producinga ferromagnetic material as a'axls of about The Crystals Proved to be Smgle defined in claim 2 in which the mixture is heated betwee crystals of the compound about 1170 c. and 1210 0.

Ca Y Fe 0 (1:05; 17:0; 6:04; 61:0; DIY 5. A method of producing a si ngle crystal of ferro- 25 magnetlc material as defined 1n claim 4 1n Wl'llCh the ma- Starting from a mixture of 25 g. of CaCO 58.4 g. terial, after heating, is cooled 50 in at least two hours.

9 Fezos of Thez and of 111203 (correspond' References Cited in the file of this patent mg to 39 mol percent of CaO, 58 mol percent of Fe O Q 2 mol percent of T110 and 1 mol percent of M 0 crys- UNITED STATES PAlENT tals having a composition 1,946,964 Cobb Feb. 13, 1934 

1. A FERROMAGNETIC MATERIAL CONSISTING ESSENTIALLY OF MIXED CRYSTALS OF COMPOUNDS HAVING A CHEMICAL COMPOSITION ACCORDING TO THE FORMULA 